soi 112 - True Arena (the best and most expensive, by a long shot)
soi 88 - Pump Up gym (I like this one at the mo.)
soi 88 - @letic (never been)
soi 94 - crusty gym (forgot the name, old equipment)
soi 102 - Amazing Muay Thai (may have changed name but same location- has full gym)

That 500bt per visit if if you are an "Orange" member which requires a one-off payment (or so I believe) of 5000bt.
An individual visit last time I went was 700bt; but I think that at that time Orange members paid only 400bt; so I don't know if non-member fees have also gone up.

Checked their website and can't say I got any wiser. 500 per visit? Strange way to get regular customers. I thought most fitness venues/gyms had like monthly, 6 month, yearly prices and a drop-in price. I normally go to the gym 5-6 times a week for about 1 hour each time. If I were to go there it would cost me 2500-3000 baht every week, and maybe a 5000 baht fee up front. Yearly fee would end up at 130,000Baht!!!! If I calculate 5 days a week! Well if this is the price at True Fitness Arena I even want go there to check out that venue!

"Two possibilities exist:
Either we are alone in the universe or we are not.
Both are equally terrifying."
Arthur C. Clarke

I have been using @Lectic for a couple of years - they are having a bit of a much needed upgrade at the moment which is good news. I am happy there, I like the staff they are friendly and there are some good classes included in your membership.